N-alkyl substituted cyclic and oligomeric perhydridosilazanes, methods of preparation thereof, and silicon nitride films formed therefrom

ABSTRACT

Novel N-alkyl substituted perhydridocyclic silazanes, oligomeric N-alkyl perhydridosilazane compounds, and N-alkylaminodihydridohalosilanes, and a method for their synthesis are provided. The novel compounds may be used to form high silicon nitride content films by thermal or plasma induced decomposition.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. application Ser. No.15/070,693, filed Mar. 15, 2016, which claims priority to U.S.Provisional Patent Application No. 62/136,916, filed Mar. 23, 2015, thedisclosures of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

The low coefficient of thermal expansion and relatively low dielectricconstant of silicon nitride (SiN) in its various forms has led to a widerange of thin films applications in semiconductor, display, photovoltaicand structural composite applications. The silicon nitride may rangefrom amorphous to various crystalline forms and can include carbon dopedmaterials or “silicon carbonitrides.” The most widely used manufacturingtechnologies for producing these films use the reaction of siliconprecursors of silane (SiH₄) or dichlorosilane (H₂SiCl₂) with ammonia(NH₃) under thermal- or plasma-assisted low-pressure chemical vapordeposition (LPCVD), sub-atmospheric pressure CVD (SACVD), or atmosphericpressure CVD (APCVD). Unfortunately, plasma assistance necessitatestemperatures in excess of 750° C. and thermal deposition processesrequire temperatures in excess of 1000° C. Plasma use also leads, inmany cases, to the incorporation of excessive concentrations of hydrogenspecies, thus limiting the quality of the resulting SiN thin films.

The use of these inorganic precursor-based thermal and plasma CVDtechnologies precludes their use for preparation on substrates that aresensitive to high temperature or highly energetic environments. Theiruse also requires stringent safety precautions as they can bepyrophoric, toxic, corrosive, or present other hazards. Further,particulate generation in the CVD reactor, resulting from the reactionof chloro or other halo-silane type chemistries with ammonia, posessignificant tool reliability problems. Therefore, there is a need in theart for an alternative to the use of silanes and chlorosilanes as rawmaterials and a need in the art for a CVD process which does not requireplasma.

Alternatives to inorganic CVD techniques have been proposed, but suchprocesses require difficult-to-access intermediates, use a high energyenvironment, and/or result in a film in which the electrical propertiesare compromised. Examples of such alternative systems include thatdescribed in U.S. Pat. No. 4,200,666 using trisilylamine ((SiH₃)₃N) andan inert gas with optional ammonia; the system of diethylsilane andammonia in an LPCVD system at 800° C., as described in A. Hochberg etal. (Mat. Res. Soc. Symp, 204, 509-514 (1991)); and the system of cyclicsilazanes and ammonia in a chemical vapor deposition (CVD) processdescribed by B. Arkles (J. Electrochemical Soc., Vol. 133, No. 1, pp.233-234 (1986)).

More recently, halide-containing precursors such as tetraiodosilane andhexachlorodisilane have been described in U.S. Pat. No. 6,586,056 and byM. Tanaka et al. (J. Electrochemical Society, 147, 2284 (2000)),respectively. Unfortunately, there are operational difficultiesassociated with the corrosiveness of the precursors, as well as withfilm contaminants and byproducts.

Another approach is the use of bis(t-butylamino)silane, which producesSiN films of reasonable quality at temperatures as low as 550° C. (J.Gumpher et al., J. Electrochem. Soc., 151, G353 (2004)) or in aplasma-assisted pulsed deposition method as described in U.S. PatentApplication Publication No. 2011/0256734. In both cases, there arecomplications with carbon contamination of films and the high energyrequirements of both the thermal and plasma regimes, which are notcompatible with substrate stability. A review of other alternativeapproaches is found in EP 2 644 609 A2, which suggests fluorinatedprecursors. While such fluorinated precursors theoretically allow lowerdeposition temperatures, the introduced fluorine frequently affectselectrical properties of silicon based structures. Thus, the need fornew SiN precursors which are able to deposit silicon nitrides at lowtemperature has still not been satisfied.

Perhydridocyclic silazanes with methyl substitution on the nitrogen havebeen contemplated in the literature by M. Rayez et al., (J. MolecularStructure, 487(3), 241-250, (1999)) and in U.S. Patent ApplicationPublication No. 2014/0051264. However, the methyl substitution does notallow a mechanism for low temperature deposition by an eliminationreaction.

SUMMARY OF THE INVENTION

An N-alkyl substituted perhydridocyclic silazane according to anembodiment of the invention has formula (1), (2), or (3):

wherein each R is independently a linear or branched alkyl group havingtwo to about ten carbon atoms, and wherein R′ is hydrogen or a linear orbranched alkyl group having two to about ten carbon atoms which isdifferent than R.

An oligomeric N-alkyl perhydridosilazane according to a secondembodiment of the invention has formula (4):

wherein R is a linear or branched alkyl group having two to about tencarbon atoms and m is an integer of about 3 to about 50.

In a further embodiment, the invention is directed toN-alkylaminodihydridohalosilanes having Formula (5):

wherein R is a linear or branched alkyl group having two to about tencarbon atoms and X is a halogen.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of preferred embodiments of thepresent invention will be better understood when read in conjunctionwith the appended drawing. For the purposes of illustrating theinvention, there is shown in the drawing an embodiment which ispresently preferred. It is understood, however, that the invention isnot limited to the precise arrangements and instrumentalities shown. Inthe drawings:

FIG. 1 is a TGA thermogram of poly(N-ethylhydridosilazane); and

FIG. 2 is a TGA thermogram of poly(N-isopropylhydridosilazane).

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a new class of precursors forsilicon nitride which may generally be described as N-alkyl substitutedperhydridocyclic silazanes, more specifically N-alkyl substitutedperhydridocyclotrisilazanes and N-alkyl substitutedperhydridocyclotetrasilazanes. These compounds have the structures shownin Formulas (1), (2), and (3):

In Formulas (1), (2), and (3), each R is independently a branched orlinear alkyl group having two to about ten carbon atoms, preferablyabout two to four carbon atoms, and is most preferably ethyl, propyl,butyl, isopropyl, or t-butyl. In Formula (3), R′ is hydrogen or abranched or linear alkyl group having two to about ten carbon atoms,preferably about two to four carbon atoms, and is most preferably ethyl,propyl, butyl, isopropyl, or t-butyl, but must be different from R.Thus, the silazane having Formula (3) contains different substituents onthe nitrogen atoms.

Salient features of the inventive compounds are that there are noorganic substituents on the silicon atoms and that organic substituents(alkyl groups) with a minimum carbon number of two are present on most,if not all, of the nitrogen atoms. The inventive compounds are stable,non-pyrophoric materials which are capable of forming silicon nitridefilms.

Exemplary compounds according to the invention include1,3,5-triethylcyclotrisilazane, 1,3,5,7-tetraethylcyclotetrasilazane,1,3,5-tri(isopropyl)cyclotrisilazane, and1,3,5-tri(tert-butyl)cyclotrisilazane, which have the followingstructures:

However, the invention is not limited to these compounds.

In contrast with known perhydridocyclic silazanes with methylsubstituents on the nitrogen atoms, the presence of organic radicals(alkyl groups) with at least two carbon atoms provides a mechanism forlow temperature deposition by elimination of ethylene, propylene, orisobutylene for ethyl, isopropyl/propyl, and t-butyl substitution,respectively. In contrast, the methyl substitution in the prior artcompounds does not allow for such a low temperature deposition byelimination.

The invention also relates to oligomeric N-alkyl perhydridosilazaneshaving Formula (4).

In Formula (4), R is a linear or branched alkyl group having two toabout ten carbon atoms, more preferably two to about four carbon atoms,most preferably ethyl, isopropyl, or t-butyl, and m is an integer ofabout 3 to 50. Exemplary compounds of this type includepoly(N-isopropylsilazane), poly(N-ethylsilazane), and1,3,5-tri-(tert-butyl)-trisilazane.

Having two hydrogen atoms on each of the silicon atoms in the oligomericcompounds is significant because it facilitates the dissociativeadsorption of hydrogen (H₂) upon interaction of the precursor with asurface. This is depicted for the case of N-ethylsilazane, which, underoptimum conditions with ammonia as a component in the carrier gasstream, could decompose cleanly according to the following equation:

Depending on the temperature and conditions, small amounts of hydrogenand carbon may be incorporated into the film to form so-called “siliconcarbonitride” as shown:

Thus, the materials of this invention offer the advantages ofvolatility, stability, non-pyrophoric nature, being free of halogens,and the ability to decompose at relatively low temperatures, preferablyabout 400° C. to about 650° C., to silicon nitride with minimalincorporation of carbon.

The invention also relates to a method for forming the N-alkylsubstituted perhydridocyclic silazanes and the oligomeric N-alkylperhydridosilazanes according to the invention. The method involvesreacting a primary alkylamine with a dihalosilane at low temperature,preferably less than about −10° C., more preferably about −30 to about−40° C. Alkylamines which may be used in the method of the inventioninclude any primary amine other than methylamine; the specific amine isselected based on the desired product. Appropriate dihalosilanes includedichlorosilane, dibromosilane, and diiodosilane. The appropriatedihalosilane may be selected by evaluating cost and depositiontemperature considerations.

For example, isopropylamine may be reacted with 1 mol of dichlorosilaneat −40° C. The intermediate (N-isopropylamino)chlorosilane is observedin reaction mixture samples withdrawn at low temperatures andimmediately analyzed by GC mass spectroscopy. The reaction mixture isthen reacted with more isopropylamine at room temperature and agitatedat that temperature for 2-24 hours. The product,1,3,5-triisopropylcyclotrisilazane, is isolated by filtration andlow-temperature vacuum distillation.

While the cyclic trisilazane is the main product observed when the aminecontains sterically hindered groups such as isopropyl, cyclictetrasilazane and oligomeric/polymeric homologs, such as those shownbelow, are also isolated when linear amines such as ethylamine areemployed as starting materials.

Accordingly, the method of the invention for producing N-alkylsubstituted perhydridocyclic silazanes also results in the formation ofthe oligomeric N-alkyl perhydridosilazanes according to the invention.

The intermediates formed during the syntheses described above also havepotential as precursors for low temperature deposition of siliconnitride films. These N-alkylaminodihydridohalosilanes have Formula (5),in which X is a halogen and R is a linear or branched alkyl group havingtwo to about ten carbon atoms, preferably about two to four carbonatoms, and is most preferably ethyl, propyl, butyl, isopropyl, ort-butyl.

These compounds include (N-ethylamino)chlorosilane,(N-isopropylamino)chlorosilane, (N-t-butylamino)chlorosilane,(N-isopropylamino) bromosilane, and (N-t-butylamino)iodosilane. In thesynthesis of the more sterically hindered t-butylcyclictrisilazane, theintermediate (N-t-butylamino)chlorosilane shown below is relativelystable and the ring-closure is accomplished by addition ofdiisobutylaluminum hydride in slight molar excess.

As previously explained, the compounds according to the invention areuseful as precursors for forming high silicon nitride content films.Accordingly, the invention also relates to high silicon nitride contentfilms formed by the thermal- or plasma-induced decomposition of theN-alkyl substituted perhydridocyclic silazanes, the oligomeric N-alkylperhydridosilazanes, and the N-alkylaminodihydridohalosilanes accordingto the invention. For the purposes of this disclosure, the phrase “highsilicon nitride content film” may be understood to mean a film whichcontains less than about 20 atom % carbon and hydrogen. It is known thatresidual hydrogen may remain on either the silicon or nitrogen atoms.This in turn is dependent on whether the deposition carrier gas isargon, nitrogen or ammonia. Typically, when the carrier gas is ammonia,carbon content is lower but hydrogen content is higher.

The invention will now be described in conjunction with the following,non-limiting examples.

Example 1: Syntheses of 1,3,5-Triethylcyclotrisilazane,1,3,5,7-Tetraethylcyclotetrasilazane, and Poly(N-ethylsilazane)

Under an argon atmosphere, a 5-liter 4-necked flask equipped with acooling bath, overhead stirrer, pot thermometer, sub-surface dip-tube,and dry-ice condenser was charged with methyl t-butyl ether (2464.2 g).The mixture was cooled to −40° C. and dichlorosilane (5.35 mol, 540.4 g)was slowly added into the flask. Ethylamine (10.70 mol, 482.4 g) wasthen added via dip-tube between −30 and −20° C. A precipitate formedimmediately and an exotherm was observed. The addition of ethylamine wascompleted over 2.5 hours. After addition was completed, the reactionmixture was slowly warmed to 25° C. and stirred at this temperature for6-12 hours. The reaction mixture as cooled to 0° C. Additionalethylamine (2.93 mol, 132.2 g) was added to the reaction mixture,maintaining temperature between 0° C. and 40° C. over 2.0 hours A secondportion of methyl t-butyl ether (308.0 g) was added to facilitateagitation. The mixture was stirred for 8 to 14 hours at room temperatureand monitored by GC. Subsequently, the reaction mixture was filtered andsolvents were removed from the filtrate under reduced pressure,maintaining pot temperature below 50° C. After the reaction mixture wasfiltered again, GC analysis indicated an estimated yield of two cyclicspecies of ˜60%. Fractional distillation of the clear filtrates afforded94.4 g (23.8% yield) of 1,3,5-triethylcyclotrisilazane: b.p. 40-42°C./0.7 mmHg, density@20° C.: 0.934, FTIR vSi-H: 2098 9(vs) and ¹HNMR(CDCl₃): 1.16 (t, 9H), 2.98 (q, 6H) and 4.72 (s, 6H). 58.2 g (15.0%yield) of 1,3,5,7-tetraethylcyclotetrasilazane: b.p. 83-5° C./1.5 mmHg,density@20° C.: 0.938 FTIR vSi-H: 2098 9(vs) and ¹HNMR (CDCl₃): 1.17 (m,9H), 3.00 (m, 6H) and 4.75 (d, 6H). A non-volatile oligomeric productwas also isolated in ˜20% yield with a molecular weight (Mn by GPC)between 1500 and 2000, corresponding to approximately 25 repeat units.IR and NMR spectra of this product were consistent with the structure ofpoly(N-ethylsilazane):

Example 2: Syntheses of 1,3,5-tri-(isopropyl)cyclotrisilazane andpoly(N-isopropylsilazane)

Under an argon atmosphere, a 5-liter 4-necked flask equipped with acooling bath, overhead stirrer, pot thermometer, sub-surface dip-tube,and dry-ice condenser was charged with methyl t-butyl ether (909 g). Themixture was cooled to −40° C. and dichlorosilane (3.0 mol, 303.0 g) wasslowly added to the pot. Isopropylamine (6.0 mol, 354.7 g) was thenadded via dip-tube between −30 and −20° C. over 2.5 hours. Afteraddition was completed, the reaction mixture was slowly warmed to 25° C.and stirred at this temperature for 8-14 hours. Additionalisopropylamine (3.0 mol, 177.4 g) was added to the reaction mixturebetween 0° C. and 40° C., followed by addition of 2^(nd) portion ofmethyl t-butyl ether (227.3 g). The mixture was stirred for 6-16 hoursand monitored by GC. The reaction mixture was filtered and solvents wereremoved from the filtrates under reduced pressure below 50° C. Thereaction mixture was filtered again and fractional distillation of theclear filtrates afforded 64.5 g (24.66) of1,3,5-tri-(N-isopropyl)cyclotrisilazane: b.p. 67-8° C./1.8 mmHg,density@20° C.: 0.919, FTIR: vS-H: 2113.6(vs) and ¹HNMR (CDCl₃): 1.29(d, 18H), 3.38 (m, 3H) and 4.80 (s, 6H). A non-volatile oligomericproduct was also observed in ˜10% yield with a molecular weight (Mn byGPC) between 800 and 1200, corresponding to approximately 12 repeatunits. IR and NMR were consistent with the structure ofpoly(N-isopropylsilazane):

Example 3: Synthesis of Tert-Butylaminochlorosilane

Under an argon atmosphere, a 5-liter 4-neck flask equipped with acooling bath, overhead stirrer, pot thermometer, addition funnel, anddry-ice condenser was charged with dichlorosilane in di-n-butyl ether(2.77 mol, 1120 g, 25 wt %). The reactor was cooled to −40° C. andtert-butylamine (5.0 mol, 364.9 g) was slowly added via addition funnelbetween −30 and −20° C. over 2.5 hours. After addition was complete, thereaction mixture was warmed to 20 to 30° C. and stirred for 24 hours atthis temperature. Product was stripped from the pot below 40° C. whilereducing pressure from 760 to 0.5 mmHg to give 1319.4 g of crudeproduct. The crude product was maintained as a solution indi-n-butylether 186.5 g (54% yield). GC-mass spec confirmed thestructure with parent and fragment ions as 122 (M⁺, 100%), 86(t-BuNHSiH⁺, 26%), FTIR vSi-H: 2199.0(s).

Example 4: Synthesis of 1,3,5-tri-(tert-butyl)cyclotrisilazane

Under an argon atmosphere, a 1-liter 4-necked flask equipped with acooling bath, magnetic stirrer, pot thermometer, addition funnel, anddry-ice distill head was charged with t-butylaminochlorosilane crudecontained in di-n-butyl ether (0.38 mol, 52.3 g) from Example 3. Themixture was cooled to −10° C. and diisobutylaluminum hydride (DIBAL-H)(0.478 mol, 67.9 g) was added via addition funnel between −5° C. and 30°C. over 3 hours. Upon completion of the addition, the pot temperaturewas slowly increased to 80° C. Pressure was reduced from 760 to 1 mmHgin order to remove lights. Redistillation of product crude under reducedpressure provided 12.5 g (14% yield) of the title compound, b.p. 74-76°C./0.4 mmHg, density@20° C.: 0.904, FTIR: vS-H: 2121.5(vs) and ¹HNMR(CDCl₃): 1.32 (s, 27H) and 4.98 (s, 6H).

Example 5: Synthesis of 1,3,5-Tri-(tert-butyl)-trisilazane

Under an argon atmosphere, a 5-liter 4-necked flask equipped with acooling bath, overhead stirrer, pot thermometer, sub-surface dip-tube,and dry-ice condenser was charged with methyl t-butyl ether (1144.3 g).The mixture was cooled to −40° C. and dichlorosilane (2.5 mol, 252.5 g)was slowly added to the pot. t-Butylamine (7.5 mol 548.6 g) was thenadded via dip-tube between −30 to −20° C. over 2.5 hours. After additionwas completed, the reaction mixture was slowly warmed to 25° C. andstirred at this temperature for 24 hours. Additional t-Butylamine (3.75mol, 274.3 g) was added to the reaction mixture between 0 and 40° C. Themixture was stirred for 3 hours and monitored by GC. The reactionmixture was filtered and solvents were removed from the filtrates underreduced pressure below 50° C. The reaction mixture was filtered againand fractional distillation of the clear filtrates afforded 107.5 g(14.0 mol) of Bis(t-butylamino)silane and 31.2 g of1,3,5-Tri-(tert-butyl)-trisilazane: b.p. 62-3° C./0.3 mmHg, density@20°C.: 0.868, FTIR: vS-H: 2140.9(vs) and 1HNMR (CDCl₃): 1.19 (s, 18H), 1.37(s, 9H) and 4.73 (s, 4H).

Example 6: Thermogravimetric Analysis

The thermal decomposition properties of poly(N-isopropylhydridosilazane)(Example 2) and poly(N-ethylhydridosilazane) (Example 1) were evaluatedby thermogravimetric analysis at 5° C./minute under nitrogen anddemonstrate the conversion to silicon nitride. As shown in FIGS. 1 and 2, analysis of both residues (after 800° exposure), excluding hydrogenwas greater than 90% Si and N, with less than 10% carbon.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. An N-alkyl substituted perhydridocyclic silazane havingFormula (2) or (3):

wherein each R is independently a branched alkyl group having three toabout ten carbon atoms and wherein R′ is hydrogen or a linear orbranched alkyl group having two to about ten carbon atoms which isdifferent than R.
 2. The N-alkyl substituted perhydridocyclic silazaneaccording to claim 1, wherein each R is independently a branched alkylgroup having three to about four carbon atoms.
 3. The N-alkylsubstituted perhydridocyclic silazane according to claim 2, wherein eachR is independently selected from the group consisting of isopropyl andt-butyl groups.